In optical communications, wavelengths and polarizations of optical signals can be multiplexed in an optical carrier. Telecommunication networks are increasingly focusing on flexibility and configurability, which require enhanced functionality of photonic integrated circuits (PICs) for optical communications, as well as compact devices. Optical devices based on multi-mode interference (MMI) have large bandwidth, polarization insensitivity, and high fabrication tolerances.
For a number of applications, it is desired to minimize a length of the MMI device manipulating the optical signal. For example, in one MMI device, an indium gallium arsenide phosphide (InGaAsP) core, such as In1-xGaxAsyP1-3 (compsition y=0.4), is arranged between an indium phosphide (InP) substrate and upper cladding.
The optical signal is concentrated in the core because the core has a high refractive index. The clading, which has a relatively low refractive index, guides the optical signal along a depth of the device. The length L of the MMI device requires a sequential number of repetitions of the beat length for the low and high wavelengths. The beat length is the length required for the polarization to rotate 360 degrees.
For example,L=M×Lπλ1=(M+1)×Lπλ2,  (1)where Lπλ1 and Lπλ2 are the beat lengths at wavelengths λ1 and λ2, respectively, and M is an integer. For an MMI device of a width W at wavelength λ, Lλ∝W2/λ, the length L is longer than 5 mm for a typical 1.27/1.29 μm wavelength splitter with W=8 μm.
However, the wavelength separation for 40/100 G Ethernet is typically 20 nm or smaller. It is a challenging to combine and separate optical signals oscillating with similar wavelengths in a small device.
For example, one MMI-based wavelength splitter/combiner is described in Yao et al., Optics Express 20 p. 18248, (2012). However, for operation of that device, wavelength separation has to be very large (such as 1.3 um and 1.55 um). Another optical manipulator is described by Jiao et al., IEEE J. Quantum Electronics, Vol. 42, No. 3, p. 266 (2006). However, a method used by that manipulator only applies to photonic crystal, and such manipulators are difficult to manufacture. Another method, described in U.S. Pat. No. 7,349,628, multiplexes or demultiplexes optical signals using an external control signal, which is not appropriate for some applications.
There is a need to manipulate optical signals with multiple wavelengths or polarizations while reducing the length and complexity of fabrication of an optical device.